There are several processes to manufacture bulk ceramic components from different ceramic materials. The processes have in common that the ceramic material can be made into shapes with thick cross sections, which is opposed to thin film ceramics that are sprayed onto a substrate and then fired into a ceramic coating.
A majority of ceramic materials are made by mixing powdered ceramic materials with various liquids to create a slurry that can be injected or poured into a mold and then dried like plaster or concrete. The dried liquid acts as a binder that temporarily holds the ceramic powder particles in place and together. The dried, molded shapes are then placed in a furnace at high temperature, the dried liquid binder is burned out, and the ceramic powder particles begin to fuse together in a solid-state diffusion process.
The particles do not melt but are essentially “welded” together wherever they make contact with each other. Since the particles do not melt, there is space between them when the sintering process is complete. This space gives rise to a final product that is porous. The amount and size of the pores depend on the size of the ceramic particles.
Another route to obtaining ceramic material does not involve ceramic powders. This route involves liquid polymer resins that are cured to a solid and then fired in a furnace where the polymer material is converted through chemical transformations into a ceramic material. This class of ceramic is known as Polymer Derived Ceramics (PDC's).
The problem with PDC's is that as they convert to a ceramic under high temperature, they go through extensive shrinking and there is a substantial mass loss that is manifested in the form of gas evolution. Typically, if the cross section of the cured polymer is too large the gas generated inside the solid during the ceramic transformation causes a pressure build up that will crack the polymer body and result in a ceramic part with multiple cracks. To avoid the formation of cracks during the ceramic transformation the evolving gas needs to diffuse through the solid polymer phase and out to the surface. The longer the pathway leading to the surface the more diffusion resistance there is and the greater the internal backpressure. Therefore, there is a maximum diffusion length that can be tolerated in a pre-ceramic polymer shape. Because of this, PDC's were typically used for thin film coatings and micro-scale component applications.
The inventor was a co-inventor on U.S. patent application Ser. No. 13/372,297, now U.S. Pat. No. 8,119,057 which issued Feb. 21, 2012, which is incorporated by reference. The patent covered the method for synthesizing bulk ceramics and structures from polymeric ceramic precursors, and required the use of open cell material to form ceramic structures, where the liquid ceramic precursors were poured into a plastic sponge material and then solidified. The resulting solid polymer block with the plastic sponge inside could then be machined into any shape then placed into a furnace and converted to ceramic. The key here was that the sponge would melt and or burn out at a low temperature and leave a connected network of internal passageways that would allow the gases to escape as the polymer converted to a ceramic. As long as the internal gasses did not have to diffuse through the solid parts more than the critical distance before encountering one of the internal open passageways, the internal pressure would not get high enough to cause cracks. Currently, this is the only method known for bulk ceramic parts to be made with these polymer derived ceramics.
However, there are drawbacks to this method. The critical diffusion length of these polymer precursors is fairly small, about 50 microns or less and therefore a sponge material has to be chosen that has pores of 50 microns or less. There are not many sources of such materials. Another requirement of the sponge material is that it must decompose at a temperature lower than the temperature at which the polymer precursors start to convert to ceramic and produce gas. This temperature is quite low, about 300 C and there are not many sponge materials that have the pore structure and thermal decomposition properties required for this ceramic producing process. Another major drawback of this method is that it takes a long time to burn out this sponge material. If the part has thick cross sections it can take days or even weeks to completely burn out the sponge material before the temperature can be increase into the ceramic production range.
Thus, the need exists for solutions to the above problems with the prior art.